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A carbide boronizing method was first developed to produce dense boron carbide- zirconium diboride (“B4C”–ZrB2) composites from zirconium carbide (ZrC) and amorphous boron powders (B) by Spark Plasma Sintering at 1800°C–2000°C. The stoichiometry of “B4C” could be tailored by changing initial boron content, which also has an influence on the processing. The self-propagating high-temperature synthesis could be ignited by 1 mol ZrC and 6 mol B at around 1240°C, whereas it was suppressed at a level of 10 mol B. B8CZrB2 ceramics sintered at 1800°C with 1 mole ZrC and 10 mole B exhibited super high hardness (40.36 GPa at 2.94 N and 33.4 GPa at 9.8 N). The primary reason for the unusual high hardness of B8CZrB2 ceramics was considered to be the formation of nano-sized ZrB2 grains.